And too how we can see to a texture very details, when to the camera are far ? And for see of near the texture to the object ?

To see the details, you gonna need more resolution/quality when you render, while more far is the camera from the ground/object.this is of course a very limited option, as Terragen canīt import shaders natively as "textures", if that is what you are referring.

How can I export in .fbx making it less heavy but still in good quality ?

if you want to see every detail of your terrain after export, as far as I know, you can't do that.TG gonna export a huge mesh if you render with huge settings, but there is many, many ways to reduce the polycount of a object in other aplications like C4D, maya, 3d max, etc.BTW, do you check the tutorial of this topic?

How can I export in .fbx making it less heavy but still in good quality ?

Fbx is actually a slightly better format to save to in terms of file size as it is a binary format and I believe does some compression of the data to reduce the file footprint.

However, much prefer .obj as it is more widely adopted and there are many external apps that can edit it outside of Terragen.

There are a few ways to reduce the file size of a Micro exporter mesh to .obj

In the micro exporter turn off normals and texture coords, the normals tend to over smooth a mesh and tex coords are not projected in a way that's useful in most cases. The size savings can be significant! You can always adjust smoothing of the normals later in your 3d app.

Each vertex is also duplicated (at least) in the saved mesh and running a delete duplicate vertex command in an external app can reduce the file size further.